Abstract : 18% Nickel Maraging steel is playing an increasingly important role in welded structures because of its superior strength. To optimize weld properties, it is necessary to understand the detailed features of the weld microstructure. The techniques of transmission electron microscopy and electron diffraction revealed microstructures and crystallographic data some of which were previously hypothesized but not observed. Dendritic and interdendritic structures are compared to that of the parent plate metal. The tungsten inert gas welding process does not appear to alter the response (precipitate formation) of the metal to heat treatment. Orientation relationships between weld phases were determined to be identical with the Kurdjumov-Sachs relationship. (Author)